VPC module in README is very out of date

Version 5.1.1 is now available, updating to this solves an unexpected attribute error but the outputs have changed.
```
module "vpc" {
source  = "terraform-aws-modules/vpc/aws"
version = "5.1.1"
```

The output:
`module.vpc.vpc_cidr_blocks` no longer exists and should be replaced with `[module.vpc.vpc_cidr_block]`